A　low　alloy　high　speed　steel　(HSS),　containing　boron,　which　could　be　used　for　roll　steel,　was　successfully　designed.　Microstructure　of　the　as-cast　low　alloy　HSS　roll　was　investigated　by　means　of　optical　microscopy　(OM),　X-ray　diffraction　(XRD),　scanning　electron　microscopy　(SEM)　and　transmission　electron　microscopy　(TEM).　Its　hardness　was　tested.　The　results　show　that　the　microstructure　of　the　low　alloy　HSS　roll　containing　boron　consists　of　lath　martensite　and　a　small　amount　of　retained　austenite,　borocarbides　of　M23(B,　C)6,　M3(B0.7C0.3)　and　(W,　Mo)2　(B,　C).　The　micro-hardness　of　the　matrix　of　the　as-cast　steel　is　820-950　HV　and　the　hardness　of　the　as-cast　high　speed　steel　roll　exceeds　64　HRC.